This invention relates to composites and in particular to composites useful as thin dielectric substrates in circuit materials, circuits, and multi-layer circuits.
As used herein, a circuit material is an article used in the manufacture of circuits and multilayer circuits, and includes circuit laminates, bond plies, conductive layers, resin coated conductive layers, and cover films. Such circuit materials comprise dielectric materials formed from a thermosetting or thermoplastic polymer. The dielectric material in a bond ply, resin covered conductive layer, or cover film can comprise a substantially flowable dielectric material, i.e., one that softens or flows during manufacture but not use of the circuit. The dielectric material in a circuit laminate (i.e., a dielectric substrate) is, in contrast, a non-flowable material that is designed to not soften or flow during manufacture or use of the circuit or multi-layer circuit.
Circuit laminates further have a conductive layer fixedly attached to a dielectric substrate layer. When a second conductive layer is disposed on the other side of the dielectric layer, the material is often referred to as a double clad circuit laminate. Patterning a conductive layer of a circuit laminate, for example by etching, provides a circuit, or in the case of a double clad circuit laminate, a double clad circuit. One or both of the conductive layers of a double clad laminate can be processed to provide circuit layers.
The aforementioned circuit materials and circuits can be combined in various configurations to provide multilayer circuits. “Multilayer circuits” as used herein refers to materials having at least two dielectric layers and at least two conductive layers, wherein at least one of the conductive layers is circuitized, and is inclusive of both subassemblies used to form finished circuits and the finished circuits themselves.
In one simple form, a multilayer circuit includes a double clad circuit and a resin coated conductive layer, wherein the dielectric material of the resin coated conductive layer is disposed on a circuit layer of the double clad circuit. In another simple form, a multilayer circuit includes a first circuit and a second circuit joined by a bond ply disposed between the circuit layer of the first circuit and the dielectric substrate of the second circuit. Typically, such multilayer circuits are formed by laminating the circuit(s) and/or circuit material(s) in proper alignment using heat and/or pressure. Bond plies can be used to provide adhesion between circuits and/or between a circuit and a conductive layer, or between two conductive layers. In place of a conductive layer bonded to a circuit with a bond ply, the multilayer circuit can include a resin coated conductive layer bonded directly to the outer layer of a circuit. In such multilayer structures, after lamination, known hole forming and plating technologies can be used to produce useful electrical pathways between conductive layers.
Circuits and circuit materials are typically divided into two classes, flexible and rigid. Flexible circuit materials generally tend to be thinner and more bendable than the so-called rigid materials. These rigid materials have a lesser degree of bendability, and the dielectric materials used to produce rigid circuit substrates typically comprise a fibrous web or other forms of reinforcement. Suitable fibrous web reinforcement can be composed of glass fibers, or alternatively, of polymeric fibers having good dielectric properties, such as aromatic polyamides (“aramids”).
While there are a variety of rigid circuit materials available today, for example FR4 epoxy glass laminates, and the like, there is a growing demand for rigid circuit materials for high performance (high frequency) applications, that is, applications operating at 1 gigahertz (GHz) or higher. High performance applications require, among other things, circuit materials having low dielectric constants for low propagation delay, lower cross talk and higher clock rates, low dissipation factor (Df) for low attenuation, better signal integrity, and lower power consumption in portables. Use of glass or aramid reinforcement materials in dielectric substrates have certain disadvantages in high performance applications. For example, glass-based dielectric substrates such as FR-4 have a relatively high dielectric constant (Dk) of 4.2 to 4.5, measured at 1 GHz). Aramid-based dielectric substrates have a relatively high moisture absorbance, which can lead to both variable dielectric constants and high dielectric loss (dissipation factor) properties in the dielectric substrate.
As the complexity of multilayer circuits increases, there is also incentive to reduce the thickness of the dielectric layers of the multilayer circuits. Thinner dielectric layers enable the addition of more layers of circuitry, keep the weight and dimensions of the circuit boards as low as possible, and allow addition of more interconnect circuitry to be incorporated into a single board. It is difficult, however, to achieve thinner dielectric layers and still maintain good mechanical and electrical properties. Very thin fibrous webs (less than or equal to 4 mils (100 micrometers)) are more prone to dimensional distortion when placed under mechanical stresses, for example the stresses associated with the manufacture of reinforced prepregs and circuit laminates. The steps of impregnating the web, curing, heating, pressing, rolling, laminating, cutting, and the like can result in dimensional distortion in the x-y plane, thinning (which results in non-uniform thickness), and even tearing. Non-woven fibrous webs are especially prone to these defects.
What is needed, therefore, is a method for the manufacture of dielectric substrates having thin (100 micrometers or less) woven or non-woven fibrous web materials incorporated therein that does not suffer from the disadvantages described above. It would also be useful if the method produced dielectric substrates having good mechanical and electrical properties, such as low moisture absorbance, low coefficients of thermal expansion (CTE) in all directions, low dielectric and dissipation factors, and/or non-flammability.